Enzymes multiple faces

Many difficulties have to be faced during the purification of tRNA methyltransferases. The major ones are ii) enzyme multiplicity, which makes it cumbersome to isolate each specific reaction product, and then difficult to evaluate specific activity and extent of purification (ii) enzyme instability (Hi) frequent contamination by ribonuclease activity (iv) possible presence of complexes with endogenous tRNA (v) unavailability of a proper tRNA substrate, as we have already discussed. To point out the problem of... 

In a somewhat related work, Nolte, Rowan et al. [13] described in 2003 a rotaxane complex that mimics the ability of processive enzymes to catalyze multiple rounds of reaction while the polymer substrate stays bound. The catalyst, which consists of a substrate-binding cavity incorporating a manganese] 111) porphyrin complex acting as the catalytic center, can oxidize alkenes complexed within the toroid cavity, provided a ligand has been attached to the outer face of the toroid to both activate the porphyrin complex and prevent it from being able to oxidize alkenes outside the cavity. 

In contrast to the multiplicity of examples of diastereotopic face discriminations in combination with other enzyme specificity discussed later, there are relatively few alcohol dehydrogenase catalyzed reductions that involve diastereotopic face selectivity alone. However, whenever a carbonyl group is present in a single chiral stereoisomer, diastereotopically face-selective reductions are possible. The reductions of the L-homocysteine derivative (47) (48), and of the (-)-oxocineole (15,4/ )-(49) (50), are two... 

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